*3.3. E*ff*ect of Pod Length*

This section investigates the effect of pod length on the drag coefficient. Three cases were examined based on the reference pod length from the Hyperloop Alpha documents. The pod length of *Lre f* = 43 m was used as a reference case to analyze the two cases of *Lre f* /2 and 2*Lre f* . Figure 11 examines the effect of pod length on the drag coefficient and the ratios of pressure and friction drags to the total drag.

As indicated in Figure 11a, the pod length does not have a significant effect on *CDp* , whereas it leads to a considerable increase in *CDf* . The variation of drag coefficient with respect to pod length follows a similar trend as the variation of drag force. The variation of drag force with pod length is presented in the sub-figure. Friction drag is generally generated along the pod surface; hence, when increasing the pod length, friction drag increases nearly proportionally. The drag acting on a shorter pod mostly depends on the pressure drag. The increase of pressure drag is mostly affected by the pressure distribution in the front and rear of the pod; hence, the extension of pod length has little effect on the pressure drag. Nevertheless, friction drag is significantly altered because it is strongly dependent on the surface area. *CDf* is nearly doubled when doubling the pod length, whereas the increase of *CDp* is negligible. Consequently, with the increase of pod length, the increase of total drag is mostly because of the increase in friction drag. To further explain the statement, the ratios of the

pressure and friction drags to the total drag are presented in Figure 11b. With *L* = *Lre f* /2, the ratio of pressure drag to friction drag is approximately 12:1. However, with *L* = 2*Lre f* , this ratio reduces to 5:1.

(**b**) Ratio of component drag to total drag

**Figure 11.** Effect of pod length: (**a**) Variation of drag and drag coefficient with respect to pod length. The results are similar to those of the steady state study by Oh et al. (2019) [5] (the difference is below 17%). (**b**) Ratio of pressure and friction drag to total drag.
